The most common malignancy occurring in humans is skin cancer, with an incidence that approaches that of all other cancer subtypes combined. Both human papillomavirus (HPV) and ultraviolet (UV) irradiation are risk factors for this disease. Immortalization by HPV may be an early stage in carcinogenesis, while UV may induce subsequent events both by inducing further mutations, as well as selecting for tumorigenic cells. HPV-16 E6/7-immortalized human keratinocytes were exquisitely sensitive to UVB-induced apoptosis compared to either vector-transduced or early passage (p7) E6/7 cells. DNA microarray analysis showed that, out of 16,000 genes monitored, three members of the Id protein family exhibit significant changes following UVB exposure; each Id was differentially regulated by UV in primary versus immortalized cells. Id proteins play important roles in differentiation, apoptosis, and tumorigenesis in a variety of other cell types. The Specific Aims will therefore test the hypothesis that Id genes play a central role in modulating keratinocyte apoptosis and differentiation induced by acute UVB exposure both in cell culture and grafted human epidermis. Specific Aim I will examine the genetic elements that are responsible for the differential regulation of Id2 and Id3 in primary and immortalized HFK. Specific Aim II will delineate the roles of Id2 and Id3 in the regulation of apoptosis in human keratinocytes. The mechanism(s) of the gene- and cell-type specific response will be studied by investigating the factors that interact preferentially with Id2 and Id3. Specific Aim III will generate a grafted human epidermis derived from keratinocytes that overexpress Id2 or Id3 to examine both the short-term (differentiation and apoptosis) and long-term (transformation) responses to UVB or topical carcinogens.